1) "There's nothing in the proposal that calls this a tax and experts we visited say rebates like the one in Medicaid never have been called taxes." I don't know who the experts you consulted are or whatever policy agendas they may have, but here are people in the health policy field who agree with the ad's contention that the rebate proposal is best described as a tax. 

"The so-called rebate isn’t a rebate at all. For a large number of Part D patients, it’s going to function as a tax. Forcing lower drug prices for a few will dramatically inflate health care costs for the rest." See:  http://www.nydailynews.com/opinion/obama-plan-force-rebates-medicare-drug-benefit-drive-costs-thousands-seniors-article-1.976435#ixzz1dhbBGZwe.

Here is Antos with Guy King, former CHIEF ACTUARY for Medicare and Medicaid, in an AEI paper, writing:

Here is Douglas Holtz-Eakin, former CBO Director, who along with Michael Ramlet writes:

"While it is easy to understand the political appeal of the rebate proposal and claims of “windfall” based on a fragmentary view of how Part D works, the policy foundations deserve closer scrutiny. In the end, not only will the cost of a new government rebate, like any tax, be borne somewhere else in the economy, but this analysis shows that seniors will also be forced to pay much higher premiums for their prescription drug plans." See: http://americanactionforum.org/topic/cost-shifting-debt-reduction-american-seniors

Here is Grace-Marie Turner of Galen Institute, who writes:

You had asked: How does Obama's rebate plan translate to a tax of at least 23 percent "on the Medicare prescription drug benefit?" Rebates would be paid by drug makers to government. That's not the same, or even close, to taxing the Part D benefit itself -- right? It's way closer than you think when you take a look at some of the projected impacts in the links and studies I indicate above. As several of the authors indicate, it amounts to squeezing the bubble. In one scenario, R&D costs take a hit, so, possibly, drug development slows and higher hospitalization costs accrue for government programs (see http://www.nber.org/papers/w8996). Or, consumers in private plans bear a bigger cost for their prescriptions (see http://mba.yale.edu/faculty/pdf/scottmortontestimony.pdf). Or, other Medicare Part D beneficiaries (i.e., non-LIS) take a hit (see  http://www.cbo.gov/ftpdocs/105xx/doc10543/08-28-MedicarePartD.pdf and  http://americanactionforum.org/sites/default/files/AAF_Part%20D%20Financial%20Impact%202%20.pdf).  Likely, the impact will be a combination of these factors, plus, perhaps, lower rates of return to shareholders in the companies, and perhaps fewer job-creation opportunities within the companies. These have impacts on government revenues the other way, downward.

3) You asked about the mandatory nature of the proposal: "That's their choice, no?" To me, that's like saying someone who lives 20 miles from their job and can't afford car-registration fee increases can always choose to stop driving and walk to work. That is a choice they can make, but not a practical one, and it is influenced directly by conditions that government creates. One of the reasons we opposed Medicare Part D's creation was precisely because it would give government more control over health care decisions in the economy. As more seniors have opted to participate in Part D (though some portions of the program are under automatic enrollment), federally administered programs have captured a larger share of the overall prescription drug market. Note the following news article: "Those filled under a Medicare Part D plan or through Medicaid represented 30 percent of all prescriptions in 2010 versus 22 percent in 2006 - a clear indication of greater dependence on government programs for medicines." See: http://health.yahoo.net/news/s/nm/us_drug_spending

4)  The only hint at who would pay this tax described in the ad is the end of the statement, "for low-income beneficiaries.” Why is that language in the statement? Is it because you expect the beneficiaries to pay the rebate? The President's rebate proposal applies to benefits for low-income individuals, though of course, it might be expanded if it were to move through Congress. But would these beneficiaries be affected? The answer is, quite possibly, which is why we raise a concern. As the Antos-King study notes:

"Focusing on rebates specific to the LIS population ignores the fact that Part D plans negotiate rebates and price discounts for drugs on behalf of their entire covered populations. Policy choices made in the Medicaid program do not transfer simply to the Part D program, which serves a broader population. Contrary to claims made by proponents, the legislation would not restore a similar requirement that previously existed for dual eligibles, and the wider application could disrupt a program that is working well. Perversely, the minimum rebate proposal could discourage Part D plans from seeking to enroll LIS individuals. In the current Part D market, manufacturers have complete flexibility to negotiate discounts with the plans. Plans with larger enrollments and those that are willing to set formulary and cost-sharing policies to favor a manufacturer's products are rewarded with higher rebates—effectively, lower prices for the pharmaceuticals used by the plan's enrollees." 

There's also the concern that the LIS portion of Part D has had a low pick-up rate anyway, and it's difficult to imagine a rebate scheme like this actually improving the rate. See, for example, the commentary by Kaiser Foundation on pick-up rates here: http://www.kff.org/medicare/upload/8094.pdf.